U.S. Pat. No. 5,081,639 teaches a laser diode assembly including a cylindrical lens. The assembly taught therein includes a diffraction-limited cylindrical lens having a numerical aperture greater than 0.5 which is used to collimate a beam from a semiconductor laser diode. A collimated beam is one which is neither converging nor diverging; i.e., the rays within the beam are travelling substantially parallel to one another. Semiconductor laser diodes are efficient sources of laser radiation, however the highly divergent beam emitted from a semiconductor laser diode presents problems in many applications. The divergence of the semiconductor laser diode's beam is caused by its exit aperture which is very narrow along one axis (the "fast" axis, which is defined to be perpendicular to the laser junction), and much wider along the perpendicular axis (the "slow" axis, which is defined to be parallel to the laser junction). These two axes correspond to the Y and X axes, as will be later explained. The cross section of the beam emitted along the fast, or Y, axis is highly divergent due to diffraction effects. In comparison, the wider aperture, defined along the X axis, emits a beam cross section that diverges only slightly.
Laser diodes, or more properly, semiconductor lasers, are generally constructed according to well known principles of semiconductor manufacturing technology. A discussion of these principles can be found in Richard R. Shurtz II, Semiconductor Lasers and LEDs in Electronics Engineers' Handbook, 3rd ed. (hereinafter "Shurtz") (Donald G. Fink and Donald Christiansen, eds. 1989,). In order to collimate the beam produced by a semiconductor laser diode, the invention taught in U.S. Pat. No. 5,081,639 teaches the mounting of a cylindrical lens optically aligned with the laser diode to provide a beam of collimated light from the Y axis of the diode.
U.S. Pat. No. 5,181,224 illustrates the use of cylindrical lenses to (inter alia) create a slowly diverging beam of light. This lens may be said to be "circularizing" and, when installed on any of a variety of laser diodes is available as the "CIRCULASER.TM." diode available from Blue Sky Research in Santa Cruz, Calif.
While the above-described laser diode assemblies are fully effective for their intended use, the method of manufacture has heretofore resulted in manufacturing inefficiencies. In any optical system, the alignment of the various optical elements is critical to the functioning of the system. This is certainly the case where a cylindrical microlens is incorporated into an optical system with a semiconductor laser diode to provide a low-cost source of collimated light. As is typical of many optical applications, there are six degrees of freedom inherent in the positioning of the lens with respect to the semiconductor laser diode, as shown in FIG. 1. Having reference to that figure, a cylindrical microlens, 100 is shown. The lens has three axes, X, Y and Z. The Z axis, 1, corresponds to the optical axis of the optical system. The X, 3, axis is transverse to the Z axis, 1, in the horizontal plane. The Y, 2, axis is also perpendicular to the Z axis but in the vertical direction.
Positioning the lens along the X, Y, and Z axes defines the first three degrees of freedom. Furthermore, the lens may be rotated about each of these axes as shown at 10, 20, and 30, and each of these rotations also defines a degree of freedom with regard to alignment of the lens in the optical system. For cylindrical lenses, placement of the lens along the X axis, 3, is not critical. This fact means that the alignment of a cylindrical microlens with respect to a semiconductor laser diode accurately requires alignment with five degrees of freedom.
It will be apparent to those of ordinary skill in the art that a mechanical translation stage providing the required five degrees of freedom is subject to considerable inaccuracies. These inaccuracies are the cumulative result of the tolerances required by any mechanical system for motion in essentially five directions.
To overcome this source of error, the manufacture of laser diode assemblies including microlenses has, to date, proceeded in a number of steps. First, a section of cylindrical microlens is mounted on a small mounting bracket which because of its resemblance to a football goal post is referred to as a "goal post." It is intended that rotation about the X and Y axes is defined by the lens' position on the goal post. After the lens is mounted on the goal post, the goal post/lens assembly is then optically positioned along the Y and Z axes, and the lens affixed to the semiconductor laser diode. In order to perform these several alignments, a laser diode, usually the diode to which the lens will ultimately be assembled, is energized and the diode's laser beam directed through the lens to a screen. The operator manipulates the lens along and about the several axes until the projected beam meets the required specifications for the assembly. This process is referred to hereinafter as "active alignment". In this manner, movement along the several axes, as well as rotation about those axes is manipulated by an operator who assembles each lens and laser diode. The entire operation is very dependent on the skill of the operator, as the optical cement utilized first to affix the lens to the goal post and finally to the diode introduces a variable into the problem. This variable is simply that the surface tension of the cement between the several elements on which it is used causes motion between those elements. This motion of course tends to misaligned the optical elements.
In contrast to the active alignment steps outlined above, passive alignment, as used herein, defines a process whereby the lens is aligned solely by means of mechanical jigs, fixtures, alignment blocks, and thereafter secured in position with respect to the diode. Passive alignment does not require the projection of a beam of light through the lens, nor indeed, manipulation of the lens with respect to beam alignment or performance. Passive alignment relies solely on the mechanical alignment of the lens with respect to the diode to achieve the required optical alignment.
The discussion on fabrication is directed to the fabrication of electro-optical devices in general, and of semiconductor lasers having in operative combination therewith at least one lens for modifying the output beam of a semiconductor laser diode. One such device of an early type is taught in U.S. Pat. No. 4,731,772, as referenced in U.S. Pat. No. 5,050,153 and this device is shown in FIGS. 2a and 2b.
Referring to FIG. 2a, the laser optical system taught by the '772 reference is shown, along with a depiction of the slow axis of the laser beam. The '772 system comprises a semiconductor laser, for instance a semiconductor laser diode 110, having in operative combination therewith a collimating lens 1102 for collimating the beam, 1101, output from laser diode 110. Further in operative combination with diode 110 and collimating lens 1101, is an astigmatism-correcting cylindrical lens 1103.
Referring now to FIG. 2b, an orthogonal view to that of FIG. 2a is shown, presenting the fast axis of output beam 1101. As shown therein, beam 1101 is not circular, but rather has a higher degree of divergence in the fast axis. Accordingly, the system according to the '772 patent is shown to be inefficient in collecting the light collected by the laser diode.
U.S. Pat. No. 5,050,153 teaches a similar device, implemented as a semiconductor laser optical head assembly, and utilizing a tilted plate for astigmatism correction in place of the cylindrical lens taught in the '772 reference. This system is shown in FIGS. 3a and 3b. Having reference to the former figure, the diode/lens assembly is shown, presenting the slow axis of the output beam, 1101. In addition to providing other functions, optical plate 1104 provides the astigmatism correction provided by a separate lens in the '772 teaching.
Referring now to FIG. 3b, the fast axis of the output beam 1101 is again shown. Again, the fast axis is more widely divergent than the slow axis, leading to loss of optical efficiency.
To overcome the loss of optical efficiencies inherent in each of these designs, U.S. Pat. No. 5,181,224 utilizes a cylindrical microlens which with one optical element circularizes and corrects the astigmatism in the output beam of a semiconductor laser diode. To obtain these advantages, the cylindrical lenses must be aligned to tolerances within 1-2 .mu.m along at least two axes. This precision alignment requires the active alignment of the lens with the diode. The resultant apparatus, e.g., the previously discussed CIRCULASER.TM., is a low-divergence, high numerical aperture, highly efficient semiconductor laser diode assembly, with properties unmatched by other laser diodes.
Indeed, the advantages accruing to the CIRCULASER.TM. are only obtainable by the use of microlenses. In optical systems of the type described in U.S. Pat. No. 5,080,706, reducing the size of the optical elements thereof is generally regarded as having positive advantages in lens fabrication and accuracy. Indeed, the performance provided by the use of microlenses, i.e. lenses not substantially larger than about 1000 .mu.m in diameter, is not attainable using macroscopic lenses.
The process whereby this apparatus is manufactured requires considerable effort on the part of skilled technicians and is a source of higher manufacturing cost or increase in manufacturing time. The apparatus taught in the '224 reference is embodied in accordance with the generally accepted principle of optical design that an optical system having fewer lens elements is both more optically efficient in collecting light emitted by the laser diode, and requires fewer alignment steps to manufacture.
The remaining problem however, is that the current manufacturing process for semiconductor laser diode assemblies including a cylindrical microlens is a labor intensive process, requiring considerable effort on the part of skilled technicians to effect the assembly of one lens to one diode.
What is needed is a methodology which will result in further substantial savings in skilled manpower currently required to accurately assemble a cylindrical microlens with a laser diode, especially a single-mode laser diode.